The present invention relates generally to acetabular cup prostheses and, more particularly, to an improved acetabular cup prosthesis of the type having a highly polished, smooth, spherical outer surface.
Acetabular cup prostheses which feature an outer shell having a smooth, spherical outer surface and a bearing insert having a smooth, spherical inner surface are known. The smooth outer surface of the outer shell fits within the natural acetabulum, and is free to move within that cavity. The smooth, inner surface of the bearing insert is fitted over the head of the femoral component of a hip prosthesis in a manner which produces three-degree freedom of movement between the bearing insert and the head. The double articulation provided by this type of prosthetic acetabular component is advantageous in providing nearly anatomical ranges of motion to prosthetic joints in certain cases, and is often less destructive of the natural acetabulum. This type of acetabular cup prosthesis is often referred to as a "bipolar" prosthesis due to the freedom of movement which exists between the femoral head and bearing insert, and the smooth outer shell and the acetabular cavity
Bipolar acetabular cup prostheses are shown and described in several prior U.S. patents, including U.S. Pat. Nos. 3,813,699 to Giliberty, 3,863,273 to Averill, 4,172,296 to D'Errico, 4,241,463 to Khovaylo, 4,408,360 to Keller, 4,619,658 to Pappas et al., 4,676,799 to Legrand, and 4,718,911 to Kenna. Several of these patents discuss the workings of such joints and the conditions under which they are most often used in replacing defective anatomical joints. To the extent that these discussions are helpful in the appreciation and understanding of the present invention, these disclosures are hereby incorporated into the present disclosure by this reference thereto.
In the design of a bipolar acetabular cup prosthesis, it is necessary to consider a number of factors, including the ease with which a particular prosthesis design can be assembled and/or disassembled under the conditions normally found in the orthopedic operating room. Moreover, the assembled prosthesis must provide some degree of resistance to dislocation and separation of components under the stresses and ranges of motion to which prosthetic joints are commonly exposed.
It is an object of the present invention to provide an improved bipolar acetabular cup prosthesis assembly which addresses the design considerations noted above.
More particularly, it is an object of the present invention to provide a bipolar acetabular cup prosthesis which provides for a high degree of resistance to dislocation and separation of the components thereof.
Another object of the present invention is to provide a bipolar acetabular cup prosthesis assembly which is relatively easy to assemble and manipulate under typical operating room conditions.
Yet another object of the present invention is to provide a bipolar acetabular cup prosthesis assembly which is easy to disassemble in the event revision of the prosthetic joint becomes necessary.
Still yet another object of the present invention is to provide a bipolar acetabular cup prosthesis assembly in which several of the components may be assembled into a sub-assembly prior to the installation procedure, thus reducing the number of individual components which must be handled by the surgeon and facilitating and simplifying the overall installation process.
These and other objects of the invention are attained in an acetabular cup prosthesis assembly which includes an outer shell, a bearing insert, means for retaining the bearing insert within the outer shell, a locking collar, and a locking ring. The outer shell of the assembly has a generally smooth hemispherical outer surface, an inner surface, an opening defined by a peripheral edge surface which joins the outer and inner surfaces, and a first circumferential groove formed in the inner surface. The bearing insert is adapted to be received within the opening in the outer shell. The insert has an outer surface, an inner surface, and an opening defined by a peripheral edge surface which joins the outer and inner surfaces. The inner surface and opening are adapted to receive the femoral head of a hip prosthesis. The opening has a first diameter which is smaller than a diameter of the femoral head, and is elastically expandable to a second diameter to allow for passage of the femoral head through the opening. The locking collar is movable from a disengaged position to an engaged position adjacent a portion of the outer surface of the bearing insert to prevent the opening from expanding to the second diameter so as to allow the femoral head to be locked into place within the insert. The locking ring is disposed within the first circumferential groove in the inner surface of the outer shell. At least a portion of the locking ring protrudes outwardly from the groove beyond the inner surface of the shell so as to interact with a portion of the locking collar to secure the collar in the engaged position adjacent the outer surface of the bearing insert. The protruding portion of the locking ring is elastically deformable inwardly into the groove so as to allow the locking collar to be selectively moved to the disengaged and engaged positions.
In one embodiment of the invention, the means for retaining the bearing insert within the opening in the outer shell comprises a second circumferential groove formed in the inner surface of the outer shell, and means formed in the outer surface of the bearing insert extending into the second groove for retaining the bearing insert within the opening in the shell. The means formed on the outer surface of the bearing insert preferably comprises an outwardly extending lip having a generally horizontal surface which extends into the second groove and which interacts with a surface of the groove to retain the bearing insert within the opening in the shell. Although the bearing insert and outer shell can be designed and dimensioned so as to allow the insert to be inserted into the outer shell by hand, it is preferred to use a mechanical means or operation to join these two components to decrease the likelihood of their unintended separation. One technique involves dimensioning the bearing insert to be slightly larger than the size that could otherwise be inserted into the outer shell, cooling the oversized bearing insert with liquid nitrogen to cause the outer dimensions thereof to shrink, and inserting the cooled bearing insert into the outer shell to allow it to expand in the assembled position.
In one embodiment of the invention, the protruding portion of the locking ring comprises a plurality of tabs formed in generally opposing relation on opposite sides of the locking ring. This embodiment further comprises means for preventing rotational movement of the locking ring so as to maintain the orientation of the tabs relative to the outer shell. The means for preventing rotational movement of the locking ring comprises a split in the ring, and means secured to the outer shell and protruding into the first circumferential groove so as to engage surfaces of the split. The means secured to the outer shell comprises a pin extending from the outer surface to the inner surface of the outer shell and into the first circumferential groove. Indicia are provided on the outer surface of the outer shell to indicate the orientation of the protruding tabs of the locking ring relative to the outer shell. A plurality of slots are provided in the locking collar to facilitate access to the plurality of tabs on the locking ring.
The protruding portion of the locking ring has an inwardly facing surface which interacts with an outwardly facing surface of the locking collar to secure the collar in the engaged position adjacent the outer surface of the bearing insert. The protruding portion of the locking ring further has an outwardly facing beveled surface which interacts with a beveled surface on the locking collar to facilitate deformation of the protruding portion inwardly into the first circumferential groove to allow the locking collar to be moved into the engaged position adjacent the outer surface of the bearing insert.
One embodiment of the present invention further comprises an extraction tool for elastically deforming the locking means when the locking collar is in the engaged position, and for interacting with the locking collar to facilitate movement of the collar to the disengaged position. The extraction tool includes a portion which is insertable within slots in the locking collar to engage and deform the protruding portions of the locking ring. A roughened, grooved, or knurled surface is provided to increase the coefficient of friction between the extraction tool and locking collar to further facilitate disengagement of the collar from the assembly.
The outer shell of the acetabular cup prosthesis of the present invention is preferably formed of a biologically compatible metal alloy, such as a cobalt-chromium alloy. Other materials, such as titanium alloys and ceramics, may also be used. The bearing insert and locking collar are preferably formed of ultra-high molecular weight polyethylene (UHMWPE). The locking ring is preferably formed of UHMWPE or high density polyethylene (HDPE).
In an especially preferred embodiment of the present invention, the outer shell, bearing insert, and locking ring may be assembled into a sub-assembly which may be separately packed and sterilized for use by the surgeon at the time of installation. This sub-assembly and the locking collar thus form a two-component system which is easily manipulated by the surgeon to simplify and facilitate the installation process.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.